Method for manufacturing photoelectric conversion device

ABSTRACT

A method for manufacturing a photoelectric conversion device including a first-conductivity-type crystalline semiconductor region, an intrinsic crystalline semiconductor region, and a second-conductivity-type semiconductor region that are stacked over an electrode is provided for a new anti-reflection structure. An interface between the electrode and the first-conductivity-type crystalline semiconductor region is flat. The intrinsic crystalline semiconductor region includes a crystalline semiconductor region, and a plurality of whiskers that are provided over the crystalline semiconductor region and include a crystalline semiconductor. The first-conductivity-type crystalline semiconductor region and the intrinsic crystalline semiconductor region are formed by a low pressure chemical vapor deposition method at a temperature higher than 550° C. and lower than 650° C. The second-conductivity-type semiconductor region is formed by a low pressure chemical vapor deposition method at a temperature lower than or equal to 550° C. or higher than or equal to 650° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion device and amethod for manufacturing the same.

2. Description of the Related Art

Recently, a photoelectric conversion device, which is a power generationmeans that generates power without carbon dioxide emissions, hasattracted attention as a countermeasure against global warming A solarcell for supplying residential power or the like, which generates powerfrom sunlight outdoors, is known as a typical example thereof. For sucha solar cell, a crystalline silicon solar cell using single crystalsilicon or polycrystalline silicon is mainly used.

An uneven structure (also referred to as a texture structure) isprovided on a surface of a solar cell using a single crystal siliconsubstrate or a polycrystalline silicon substrate in order to reducesurface reflection. The uneven structure provided on the surface of thesilicon substrate is formed by etching the silicon substrate with analkaline solution such as an aqueous sodium hydroxide solution. Theetching rate by the alkaline solution varies depending on a crystalplane orientation of silicon; when a silicon substrate with a (100)plane is used for example, a pyramidal uneven structure is formed.

Although the above uneven structure can reduce surface reflection of thesolar cell, the alkaline solution used for etching causes contaminationof the silicon semiconductor and is therefore not appropriate. Inaddition, since etching characteristics considerably vary depending onthe concentration or temperature of the alkaline solution, it isdifficult to form the uneven structure on the surface of the siliconsubstrate with high reproducibility. For the difficulty, a combinationmethod of a laser processing technique and chemical etching is disclosed(for example, see Patent Document 1).

On the other hand, in a solar cell whose photoelectric conversion layeris formed using a semiconductor thin film of silicon or the like, it isdifficult to form an uneven structure on a surface of the silicon thinfilm by above etching using an alkaline solution.

Reference

[Patent Document 1] Japanese Published Patent Application No.2003-258285

SUMMARY OF THE INVENTION

In any case, the method in which the silicon substrate itself is etchedto form the uneven structure on the surface of the silicon substrate isnot favorable because the method has a problem in controllability of theuneven shape and affects the characteristics of the solar cell. Inaddition, since the alkaline solution and a large amount of rinse waterare needed for etching of the silicon substrate and it is necessary topay attention to the contamination of the silicon substrate, the methodis also not favorable in terms of productivity.

Thus, an object of an embodiment of the present invention is to providea photoelectric conversion device having a novel anti-reflectionstructure.

One feature of an embodiment of the present invention is to form anuneven structure on a surface of a semiconductor by crystal growth ofthe same or different kind of semiconductor and thereby providing ananti-reflection structure. This eliminates the need for etching asurface of a semiconductor substrate or a semiconductor film.

For example, by providing a semiconductor layer including a plurality ofprotrusions on a light incident plane side of a photoelectric conversiondevice, surface reflection can be considerably reduced. Such a structurecan be formed by a vapor deposition method; therefore, the contaminationof the semiconductor is not caused.

By a vapor deposition method, a semiconductor layer including aplurality of whiskers as an uneven structure can be grown, whereby ananti-reflection structure of the photoelectric conversion device can beformed.

One embodiment of the present invention is a photoelectric conversiondevice including a first-conductivity-type crystalline semiconductorregion being provided over a conductive layer; a crystallinesemiconductor region being provided over the first-conductivity-typecrystalline semiconductor region, having an uneven surface by includinga plurality of whiskers including a crystalline semiconductor, andhaving a concentration gradient of an impurity element imparting thefirst conductivity type; and a second-conductivity-type crystallinesemiconductor region being provided to cover an uneven surface of thecrystalline semiconductor region having the uneven surface, the secondconductivity type being opposite to the first conductivity type.

Another embodiment of the present invention is a photoelectricconversion device including a first-conductivity-type crystallinesemiconductor region, an intrinsic crystalline semiconductor region, anda second-conductivity-type semiconductor region that are stacked over anelectrode. An interface between the electrode and thefirst-conductivity-type crystalline semiconductor region is flat. Theintrinsic crystalline semiconductor region includes a crystallinesemiconductor region, and a plurality of whiskers that are provided overthe crystalline semiconductor region and include a crystallinesemiconductor. In other words, the intrinsic crystalline semiconductorregion includes the plurality of whiskers; thus, a surface of thesecond-conductivity-type semiconductor region is uneven, and aninterface between the intrinsic semiconductor region and thesecond-conductivity-type semiconductor region is uneven. Further, aconcentration gradient of an impurity element imparting the firstconductivity type is formed from the first-conductivity-type crystallinesemiconductor region toward the intrinsic crystalline semiconductorregion.

Another embodiment of the present invention is a photoelectricconversion device including a first-conductivity-type crystallinesemiconductor region, an intrinsic crystalline semiconductor region, anda second-conductivity-type semiconductor region that are stacked over anelectrode. An interface between the electrode and thefirst-conductivity-type crystalline semiconductor region is flat. Thefirst-conductivity-type crystalline semiconductor region includes acrystalline semiconductor region including an impurity element impartingthe first conductivity type, and a plurality of whiskers that areprovided over the crystalline semiconductor region and include acrystalline semiconductor including an impurity element imparting thefirst conductivity type. In other words, the first-conductivity-typecrystalline semiconductor region includes the plurality of whiskers;thus, a surface of the second-conductivity-type semiconductor region isuneven, and an interface between the first-conductivity-type crystallinesemiconductor region and the intrinsic crystalline semiconductor regionis uneven. Further, a concentration gradient of an impurity elementimparting the first conductivity type is formed from thefirst-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region.

Note that in the above photoelectric conversion device, thefirst-conductivity-type crystalline semiconductor region is one of ann-type semiconductor region and a p-type semiconductor region, and thesecond-conductivity-type semiconductor region is the other of the n-typesemiconductor region and the p-type semiconductor region.

In the photoelectric conversion device, the second-conductivity-typesemiconductor region can be formed using an amorphous semiconductor, acrystalline semiconductor, or a semiconductor material in which anamorphous semiconductor and a crystalline semiconductor are mixed.

In the photoelectric conversion device, a material of the intrinsiccrystalline semiconductor region and a material of thesecond-conductivity-type semiconductor region may have different bandgaps. In the photoelectric conversion device, a material of thesecond-conductivity-type semiconductor region may have a larger band gapthan the intrinsic crystalline semiconductor region.

One embodiment of the present invention is a photoelectric conversiondevice including a third-conductivity-type semiconductor region, anintrinsic semiconductor region, and a fourth-conductivity-typesemiconductor region that are stacked over the second-conductivity-typesemiconductor region. Note that the band gap of the intrinsiccrystalline semiconductor region is different from the band gap of theintrinsic semiconductor region.

Note that in the above photoelectric conversion device, thefirst-conductivity-type crystalline semiconductor region and thethird-conductivity-type semiconductor region are one of an n-typesemiconductor region and a p-type semiconductor region, and thesecond-conductivity-type semiconductor region and thefourth-conductivity-type semiconductor region are the other of then-type semiconductor region and the p-type semiconductor region.

Directions of axes of the plurality of whiskers which are provided inthe first-conductivity-type crystalline semiconductor region or theintrinsic crystalline semiconductor region may be the direction normalto the electrode. Alternatively, the directions of axes of the pluralityof whiskers which are provided in the first-conductivity-typecrystalline semiconductor region or the intrinsic crystallinesemiconductor region may be varied.

The electrode includes at least one of a conductive layer and the mixedlayer. The electrode may include both the conductive layer and the mixedlayer. The whole electrode may be the mixed layer. Note that the mixedlayer is not necessarily provided. The conductive layer can be formedusing a metal element which forms silicide by reacting with silicon.Alternatively, the conductive layer can be formed with a stacked layerstructure including a layer which is formed using a material having highconductivity such as a metal element typified by platinum, aluminum, orcopper, and a layer which is formed using a metal element which formssilicide by reacting with silicon.

The mixed layer includes a metal element and silicon. The mixed layermay include silicon and a metal element which is included in theconductive layer. In the case where the conductive layer is formed usinga metal element which forms silicide by reacting with silicon, the mixedlayer may be formed of silicide.

In the photoelectric conversion device, the first-conductivity-typecrystalline semiconductor region or the intrinsic crystallinesemiconductor region includes a plurality of whiskers; thus, lightreflectance at the surface of the second-conductivity-type semiconductorregion can be reduced. In addition, since the photoelectric conversionlayer absorbs light incident on the photoelectric conversion layer owingto a light-trapping effect, characteristics of the photoelectricconversion device can be improved.

One embodiment of the present invention is a method for manufacturing aphotoelectric conversion device, including the steps of forming afirst-conductivity-type crystalline semiconductor region by a lowpressure chemical vapor deposition method (hereinafter, also referred toas a low pressure CVD method or an LPCVD method) using a deposition gascontaining silicon and a gas imparting the first conductivity type as asource gas at a temperature higher than 550° C. and lower than 650° C.over a conductive layer; forming an intrinsic crystalline semiconductorregion that includes a crystalline semiconductor region and a pluralityof whiskers including a crystalline semiconductor by a low pressure CVDmethod using a deposition gas containing silicon as a source gas at atemperature higher than 550° C. and lower than 650° C. over thefirst-conductivity-type crystalline semiconductor region, and alsomoving an impurity element imparting the first conductivity type fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region; and forming asecond-conductivity-type semiconductor region by a low pressure CVDmethod using a deposition gas containing silicon and a gas imparting thesecond conductivity type as a source gas at a temperature lower than orequal to 550° C. or higher than or equal to 650° C. over the intrinsiccrystalline semiconductor region. In the formation of thefirst-conductivity-type crystalline semiconductor region, a mixed layermay be formed between the conductive layer and thefirst-conductivity-type crystalline semiconductor region.

One embodiment of the present invention is a method for manufacturing aphotoelectric conversion device, including the steps of forming afirst-conductivity-type crystalline semiconductor region that includes acrystalline semiconductor region and a plurality of whiskers including acrystalline semiconductor by a low pressure CVD method using adeposition gas containing silicon and a gas imparting the firstconductivity type as a source gas at a temperature higher than 550° C.and lower than 650° C. over a conductive layer; forming an intrinsiccrystalline semiconductor region by a low pressure CVD method using adeposition gas containing silicon as a source gas at a temperaturehigher than 550° C. and lower than 650° C. over thefirst-conductivity-type crystalline semiconductor region, and alsomoving an impurity element imparting the first conductivity type fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region; and forming asecond-conductivity-type semiconductor region by a low pressure CVDmethod using a deposition gas containing silicon and a gas imparting thesecond conductivity type as a source gas at a temperature lower than orequal to 550° C. or higher than or equal to 650° C. over the intrinsiccrystalline semiconductor region. In the formation of thefirst-conductivity-type crystalline semiconductor region, a mixed layermay be formed between the conductive layer and thefirst-conductivity-type crystalline semiconductor region.

In the above embodiment, by forming the first-conductivity-typecrystalline semiconductor region and the intrinsic crystallinesemiconductor region by a low pressure CVD method at a temperaturehigher than 550° C. and lower than 650° C., the intrinsic crystallinesemiconductor region can include a crystalline semiconductor regionincluding a plurality of whiskers including a crystalline semiconductorand a crystalline semiconductor region. At the same time, the impurityelement imparting the first conductivity type is moved from thefirst-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region, whereby a concentrationgradient of an impurity element imparting the first conductivity type isformed from the first-conductivity-type crystalline semiconductor regiontoward the intrinsic crystalline semiconductor region. Note that thetemperature may be outside the above range as long as the temperatureallows the crystalline semiconductor region including a plurality ofwhiskers to be formed. In addition, another condition may be employed aslong as the condition allows a plurality of whiskers to be formed.

By forming the second-conductivity-type semiconductor region by a lowpressure CVD method at a temperature lower than or equal to 550° C. orhigher than or equal to 650° C., the second-conductivity-typesemiconductor region can be formed (i.e. deposited) while whiskers arenot grown. Note that the temperature may be outside the above range aslong as the temperature allows the semiconductor region to be depositedwhile whiskers are not grown. In addition, another condition may beemployed as long as the condition allows the semiconductor region to bedeposited while whiskers are not grown.

Silicon hydride, silicon fluoride, or silicon chloride may be used forthe deposition gas containing silicon. The gas imparting the firstconductivity type is one of diborane and phosphine, and the gasimparting the second conductivity type is the other of the diborane andthe phosphine.

By forming a photoelectric conversion device over the conductive layerwhich is formed using a metal element which forms silicide by reactingwith silicon by a low pressure CVD method, the first-conductivity-typecrystalline semiconductor region which includes a plurality of whiskersor the intrinsic crystalline semiconductor region which includes aplurality of whiskers can be formed.

Note that in this specification, an “intrinsic semiconductor” refers tonot only a so-called intrinsic semiconductor in which the Fermi levellies in the middle of the band gap, but a substantially intrinsicsemiconductor in which the concentration of an impurity imparting p-typeor n-type conductivity is 1×10²⁰ cm⁻³ or lower and photoconductivity is100 times or more as high as the dark conductivity. This intrinsicsemiconductor may include an impurity element belonging to Group 13 orGroup 15 of the periodic table. Accordingly, the problems can be solvedeven with the use of a semiconductor having n-type or p-typeconductivity as well as the use of the intrinsic semiconductor, and thusanother semiconductor having a similar effect can be used.

According to an embodiment of the present invention, the surface of thesecond-conductivity-type semiconductor region is uneven, whereby thecharacteristics of the photoelectric conversion device can be improved.In other words, by providing a group of whiskers for a plane on a lightincident side of the intrinsic crystalline semiconductor region, surfacereflection can be reduced.

By forming a concentration gradient of the impurity element impartingthe first conductivity type from the first-conductivity-type crystallinesemiconductor region toward the intrinsic crystalline semiconductorregion, a decrease in short-circuit current in the photoelectricconversion cell can be prevented. In other words, even if the lifetimeof minor carriers is shortened because of defects in the crystallinesemiconductor region including a group of whiskers, a short-circuitcurrent can be prevented from decreasing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 2 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 3 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIGS. 4A to 4C are cross-sectional views illustrating a method formanufacturing a photoelectric conversion device.

FIG. 5 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 6 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 7 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 8 is a cross-sectional view illustrating a photoelectric conversiondevice.

FIG. 9 is a graph showing light regular reflectance.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed with reference to the drawings. Note that the invention is notlimited to the following description, and it will be readily appreciatedby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Thus, the present invention should not be construed as being limited tothe following description of the embodiments and example. In descriptionwith reference to the drawings, in some cases, the same referencenumerals are used in common for the same portions in different drawings.Further, in some cases, the same hatching patterns are applied tosimilar parts, and the similar parts are not necessarily designated byreference numerals.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the scale of each structure is notnecessarily limited to that illustrated in the drawings.

Note that terms such as first, second, and third in this specificationare used in order to avoid confusion among components, and the terms donot limit the components numerically. Therefore, for example, the term“first” can be replaced with the term “second”, “third”, or the like asappropriate.

Embodiment 1

In this embodiment, a structure of a photoelectric conversion devicewhich is one embodiment of the present invention is described withreference to FIG. 1, FIG. 2, FIG. 3, and FIGS. 4A to 4C.

A photoelectric conversion device described in this embodiment includesa first-conductivity-type crystalline semiconductor region 107 providedover a conductive layer, an intrinsic crystalline semiconductor region109 which is provided over the first-conductivity-type crystallinesemiconductor region 107, has an uneven surface by including a pluralityof whiskers including a crystalline semiconductor, and has aconcentration gradient of an impurity element imparting the firstconductivity type, and a second-conductivity-type crystallinesemiconductor region 111 provided to cover the uneven surface of thecrystalline semiconductor region 109 having the uneven surface. Thesecond conductivity type is opposite to the first conductivity type.

FIG. 1 is a photoelectric conversion device including a substrate 101,an electrode 103, the first-conductivity-type crystalline semiconductorregion 107, the intrinsic crystalline semiconductor region 109, thesecond-conductivity-type crystalline semiconductor region 111, and aninsulating layer 113. The second conductivity type is opposite to thefirst conductivity type. The first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 function as a photoelectric conversion layer 106.

In this embodiment, the electrode 103 includes at least one of theconductive layer 104 and the mixed layer 105. The electrode 103 mayinclude both the conductive layer 104 and the mixed layer 105. The wholeelectrode 103 may be the mixed layer 105. Note that the mixed layer 105is not necessarily provided. When the electrode 103 includes theconductive layer 104, the conductivity of the electrode 103 can beenhanced. An interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 is flat.The intrinsic crystalline semiconductor region 109 includes a flatportion and a plurality of whiskers (a group of whiskers). In otherwords, the interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 is flatwhile a surface of the second-conductivity-type semiconductor region isuneven. In addition, an interface between the intrinsic crystallinesemiconductor region 109 and the second-conductivity-type crystallinesemiconductor region 111 is uneven, and an interface between thesecond-conductivity-type crystalline semiconductor region 111 and theinsulating layer 113 is uneven.

The first-conductivity-type crystalline semiconductor region 107 is oneof an n-type semiconductor region and a p-type semiconductor region, andthe second-conductivity-type crystalline semiconductor region 111 is theother of the n-type semiconductor region and the p-type semiconductorregion. In this embodiment, a p-type crystalline semiconductor layer andan n-type crystalline semiconductor layer are used as thefirst-conductivity-type crystalline semiconductor region 107 and thesecond-conductivity-type crystalline semiconductor region 111,respectively; however, the p-type conductivity and the n-typeconductivity may be interchanged with each other.

As the substrate 101, a glass substrate typified by an aluminosilicateglass substrate, a barium borosilicate glass substrate, and analuminoborosilicate glass substrate, a sapphire substrate, a quartzsubstrate, or the like can be used. Alternatively, a substrate in whichan insulating film is formed over a metal substrate such as a stainlesssteel substrate may be used. In this embodiment, a glass substrate isused as the substrate 101.

The conductive layer 104 is formed using a metal element which formssilicide by reacting with silicon. Alternatively, the conductive layer104 may have a stacked layer structure which includes a layer formedusing a metal element having high conductivity typified by platinum,aluminum, copper, titanium, and an aluminum alloy to which an elementwhich improves heat resistance, such as silicon, titanium, neodymium,scandium, or molybdenum, is added on the substrate 101 side; and a layerformed using a metal element which forms silicide by reacting withsilicon on the first-conductivity-type crystalline semiconductor region107 side. Examples of the metal element which forms silicide by reactingwith silicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, cobalt, and nickel.

The mixed layer 105 may be formed using silicon and the metal elementwhich is included in the conductive layer 104. Note that in the casewhere the mixed layer 105 is formed using silicon and the metal elementincluded in the conductive layer 104, active species of a source gas aresupplied to a portion being deposited in the formation of thefirst-conductivity-type crystalline semiconductor region by heat appliedin an LPCVD method, and thus silicon is diffused into the conductivelayer 104 to faun the mixed layer 105.

In the case where the conductive layer 104 is formed using a metalelement which forms silicide by reacting with silicon, silicideincluding the metal element is formed in the mixed layer 105. Thesilicide is typically one or more of zirconium silicide, titaniumsilicide, hafnium silicide, vanadium silicide, niobium silicide,tantalum silicide, chromium silicide, molybdenum silicide, cobaltsilicide, and nickel silicide. Alternatively, an alloy layer of siliconand a metal element which forms silicide is formed.

In the case where the mixed layer 105 is provided between the conductivelayer 104 and the first-conductivity-type crystalline semiconductorregion 107, resistance at an interface between the conductive layer 104and the first-conductivity-type crystalline semiconductor region 107 canbe reduced; therefore, series resistance can be reduced as compared tothe case where the first-conductivity-type crystalline semiconductorregion 107 is directly stacked over the conductive layer 104. Inaddition, the adhesiveness between the conductive layer 104 and thefirst-conductivity-type crystalline semiconductor region 107 can beincreased. As a result, yield of the photoelectric conversion device canbe improved.

Note that the conductive layer 104 may have a foil shape, a plate shape,or a net shape. With such a shape, the conductive layer 104 can hold itsshape by itself, and the substrate 101 is therefore not essential. Forthis reason, cost can be reduced. In addition, when the conductive layer104 has a foil shape, a flexible photoelectric conversion device can bemanufactured.

The first-conductivity-type crystalline semiconductor region 107 istypically formed using a semiconductor to which an impurity elementimparting a conductivity type is added. Silicon is suitable for asemiconductor material, in terms of productivity, a price, or the like.When silicon is used as the semiconductor material, phosphorus orarsenic, which imparts n-type conductivity, or boron, which impartsp-type conductivity, is used as the impurity element imparting aconductivity type. Here, the first-conductivity-type crystallinesemiconductor region 107 is formed using a p-type crystallinesemiconductor.

Note that although the first conductivity type is p-type in thisembodiment, the first conductivity type may be n-type.

The intrinsic crystalline semiconductor region 109 includes acrystalline semiconductor region 109 a and a group of plural whiskers109 b including a crystalline semiconductor over the crystallinesemiconductor region 109 a. Note that the interface between thecrystalline semiconductor region 109 a and the whisker 109 b is unclear.A plane that is in the same level as the bottom of the deepest valley ofthe valleys formed among whiskers 109 b and is parallel to a surface ofthe electrode 103 is regarded as the interface between the crystallinesemiconductor region 109 a and the whisker 109 b. The photoelectricconversion device in this embodiment includes one or more of the abovewhiskers.

The crystalline semiconductor region 109 a covers thefirst-conductivity-type crystalline semiconductor region 107. Inaddition, the whisker 109 b is a whisker-like protrusion, and aplurality of the protrusions are dispersed. Note that the whisker 109 bmay have a column-like shape such as a cylinder or a prism, or aneedle-like shape such as a cone or a pyramid. The top of the whisker109 b may be rounded. The diameter of the whisker 109 b is greater thanor equal to 100 nm and less than or equal to 10 μm, preferably greaterthan or equal to 500 nm and less than or equal to 3 μm. Further, thelength along the axis of the whisker 109 b is greater than or equal to300 nm and less than or equal to 20 μm, preferably greater than or equalto 500 nm and less than or equal to 15 μm.

Note that the length h along the axis of the whisker 109 b is thedistance between the top (or the center of the top surface) of thewhisker 109 b and the crystalline semiconductor region 109 a along theaxis running through the top (or the center of the top surface) of thewhisker 109 b. The thickness of the intrinsic crystalline semiconductorregion 109 is the sum of the thickness of the crystalline semiconductorregion 109 a and the length of a line running from the top of thewhisker 109 b perpendicularly to the crystalline semiconductor region109 a (i.e., the height of the whisker). The diameter of the whisker 109b refers to a length of a longer axis of a transverse cross-sectionalshape at the interface between the crystalline semiconductor region 109a and the whisker 109 b.

Note that the direction in which the whisker 109 b extends from thecrystalline semiconductor region 109 a is referred to as a longitudinaldirection. A cross-sectional shape along the longitudinal direction isreferred to as a longitudinal cross-sectional shape. The shape of theplane normal to the longitudinal direction is referred to as atransverse cross-sectional shape.

In FIG. 1, the longitudinal directions of the whiskers 109 b included inthe intrinsic crystalline semiconductor region 109 extend in onedirection, for example, the direction normal to the surface of theelectrode 103. Note that the longitudinal direction of the whisker 109 bmay be substantially the same as the direction normal to the surface ofthe electrode 103. It is preferable that the difference between theangles of the two directions be typically within 5°.

Note that although the longitudinal directions of the whiskers 109 bincluded in the intrinsic crystalline semiconductor region 109 extend inone direction, for example, the direction normal to the surface of theelectrode 103 in FIG. 1, the longitudinal directions of the whiskers maybe varied. Typically, the intrinsic crystalline semiconductor region 109may include a whisker whose longitudinal direction is substantially thesame as the direction normal to the surface of the electrode 103 and awhisker whose longitudinal direction is different from the directionnormal to the surface of the electrode 103.

The second-conductivity-type crystalline semiconductor region 111 isformed using an n-type crystalline semiconductor. Note thatsemiconductor materials which can be used for thesecond-conductivity-type crystalline semiconductor region 111 aresimilar to those for the first-conductivity-type crystallinesemiconductor region 107.

In this embodiment, an interface between the intrinsic crystallinesemiconductor region 109 and the second-conductivity-type crystallinesemiconductor region 111 and the surface of the second-conductivity-typecrystalline semiconductor region 111 are uneven. Therefore, reflectanceof light incident on the second-conductivity-type crystallinesemiconductor region 111 can be reduced. Further, the light incident onthe photoelectric conversion layer 106 is efficiently absorbed by thephotoelectric conversion layer 106 owing to a light-trapping effect;thus, the characteristics of the photoelectric conversion device can beimproved.

Note that a concentration gradient of an impurity element imparting thefirst conductivity type is preferably formed from thefirst-conductivity-type crystalline semiconductor region 107 toward theintrinsic crystalline semiconductor region 109, which are illustrated inFIG. 1. In other words, a concentration gradient of the impurity elementis preferably formed between the crystalline semiconductor region 107and the crystalline semiconductor region 109 (the region is alsoreferred to as a contact portion). Note that since the interface betweenthe crystalline semiconductor region 107 and the crystallinesemiconductor region 109 is not clear, an embodiment of the presentinvention includes the case where the concentration gradient is in thecrystalline semiconductor region 107, the case where the concentrationgradient is in the crystalline semiconductor region 109, the case wherethe concentration gradient is in the both regions, and the case wherethe concentration gradient is in another region.

FIG. 6 is an enlarged view of a whisker in FIG. 1. As in FIG. 6, part ofthe impurity element (X) imparting the first conductivity type includedin the crystalline semiconductor region 107 moves from the crystallinesemiconductor region 107 toward the crystalline semiconductor region109, whereby a concentration gradient is formed in which the impurityelement (X) is increased from the intrinsic crystalline semiconductorregion 109 side toward the first-conductivity-type crystallinesemiconductor region 107. In other words, the impurity elementconcentration in the crystalline semiconductor region 107 is higher thanthat in the crystalline semiconductor region 109.

By thus forming the concentration gradient of the impurity element fromthe crystalline semiconductor region 107 toward the crystallinesemiconductor region 109, a decrease in short-circuit current in thephotoelectric conversion device can be prevented. In other words, evenif the lifetime of minor carriers is shortened because of defects in thecrystalline semiconductor region including a group of whiskers, ashort-circuit current can be prevented from decreasing.

Note that the concentration gradient is not limited to a continuouschange of the concentration of the impurity element. For example, aregion whose concentration of the impurity element is higher than thatof the crystalline semiconductor region 109 and lower than that of thecrystalline semiconductor region 107 may be provided between thecrystalline semiconductor region 107 and the crystalline semiconductorregion 109.

Note that whereas the interface between the first-conductivity-typecrystalline semiconductor region 107 and the intrinsic crystallinesemiconductor region 109 is flat in FIG. 1, an interface between afirst-conductivity-type crystalline semiconductor region 108 and theintrinsic crystalline semiconductor region 109 may be uneven asillustrated in FIG. 2. The first-conductivity-type crystallinesemiconductor region 108 illustrated in FIG. 2 includes a crystallinesemiconductor region 108 a including an impurity element imparting thefirst conductivity type and a group of plural whiskers 108 b including acrystalline semiconductor including the impurity element imparting thefirst conductivity type over the crystalline semiconductor region 108 a.Note that the interface between the crystalline semiconductor region 108a and the whisker 108 b is unclear. A plane that is in the same level asthe bottom of the deepest valley of the valleys formed among whiskers108 b and is parallel to a surface of the electrode 103 is regarded asthe interface between the crystalline semiconductor region 108 a and thewhisker 108 b.

The whisker 108 b is a whisker-like protrusion, and a plurality of theprotrusions are dispersed. Note that the whisker 108 b may have acolumn-like shape such as a cylinder or a prism, or a needle-like shapesuch as a cone or a pyramid. The top of the whisker 108 b may berounded.

The longitudinal directions of the whiskers 108 b included in thefirst-conductivity-type crystalline semiconductor region 108 extend inone direction, for example, the direction normal to the surface of theelectrode 103. Note that the longitudinal direction of the whisker 108 bmay be substantially the same as the direction normal to the surface ofthe electrode 103. In that case, it is preferable that the differencebetween the angles of the two directions be typically within 5°.

Note that although the longitudinal directions of the whiskers 108 bincluded in the first-conductivity-type crystalline semiconductor region108 extend in one direction, for example, the direction normal to thesurface of the electrode 103 in FIG. 2, the longitudinal directions ofthe whiskers may be varied. Typically, the first-conductivity-typecrystalline semiconductor region 108 may include a whisker whoselongitudinal direction is substantially the same as the direction normalto the surface of the electrode 103 and a whisker whose longitudinaldirection is different from the direction normal to the surface of theelectrode 103.

In the photoelectric conversion device illustrated in FIG. 2, theinterface between the first-conductivity-type crystalline semiconductorregion 108 and the intrinsic crystalline semiconductor region 109, theinterface between the intrinsic crystalline semiconductor region 109 andthe second-conductivity-type crystalline semiconductor region 111, andthe surface of the second-conductivity-type crystalline semiconductorregion 111 are uneven. Therefore, the reflectance of light incident onthe second-conductivity-type crystalline semiconductor region 111 can bereduced. In addition, light incident on the photoelectric conversionlayer 106 is efficiently absorbed by the photoelectric conversion layer106 owing to a light-trapping effect. Accordingly, the characteristicsof the photoelectric conversion device can be improved.

Note that a concentration gradient of an impurity element imparting thefirst conductivity type is preferably formed from thefirst-conductivity-type crystalline semiconductor region 108 toward theintrinsic crystalline semiconductor region 109, which are illustrated inFIG. 2. In other words, a concentration gradient of the impurity elementis preferably formed between the crystalline semiconductor region 108and the crystalline semiconductor region 109 (the region is alsoreferred to as a contact portion). Note that since the interface betweenthe crystalline semiconductor region 108 and the crystallinesemiconductor region 109 is not clear, an embodiment of the presentinvention includes the case where the concentration gradient is in thecrystalline semiconductor region 108, the case where the concentrationgradient is in the crystalline semiconductor region 109, the case wherethe concentration gradient is in the both regions, and the case wherethe concentration gradient is in another region.

FIG. 7 is an enlarged view of a whisker in FIG. 2. As in FIG. 7, part ofthe impurity element (X) imparting the first conductivity type includedin the crystalline semiconductor region 108 moves from the crystallinesemiconductor region 108 toward the crystalline semiconductor region109, whereby a concentration gradient is formed in which the impurityelement (X) is increased from the intrinsic crystalline semiconductorregion 109 side toward the first-conductivity-type crystallinesemiconductor region 107. In other words, the impurity elementconcentration in the crystalline semiconductor region 108 is higher thanthat in the crystalline semiconductor region 109.

By thus forming the concentration gradient of the impurity element fromthe crystalline semiconductor region 108 toward the crystallinesemiconductor region 109, a decrease in short-circuit current in thephotoelectric conversion device can be prevented. In other words, evenif the lifetime of minor carriers is shortened because of defects in thecrystalline semiconductor region including a group of whiskers, ashort-circuit current can be prevented from decreasing.

Note that the concentration gradient is not limited to a continuouschange of the concentration of the impurity element. For example, aregion whose concentration of the impurity element is higher than thatof the crystalline semiconductor region 109 and lower than that of thecrystalline semiconductor region 108 may be provided between thecrystalline semiconductor region 108 and the crystalline semiconductorregion 109.

Note that the insulating layer 113 which serves as a protective layerand has an anti-reflection function is preferably formed over an exposedsurface of the second-conductivity-type crystalline semiconductor region111.

For the insulating layer 113, a material whose refractive index isbetween the refractive indices of a light incident plane of thesecond-conductivity-type crystalline semiconductor region 111 and air isused. In addition, a material which transmits light with a predeterminedwavelength is used so that incidence of light on thesecond-conductivity-type crystalline semiconductor region 111 is notinterrupted. The use of such a material can prevent reflection at thelight incidence plane of the second-conductivity-type crystallinesemiconductor region 111. Note that as such a material, silicon nitride,silicon nitride oxide, or magnesium fluoride can be given, for example.

Note that a light-transmitting conductive layer may be formed over thesecond-conductivity-type crystalline semiconductor region 111. Thelight-transmitting conductive layer is a conductive layer formed usingan indium oxide-tin oxide alloy (ITO), zinc oxide (ZnO), tin oxide(SnO₂), zinc oxide containing aluminum, or the like.

Note that as illustrated in FIG. 3, a grid electrode 115 for reducingresistance may be provided on the second-conductivity-type crystallinesemiconductor region 111 or the insulating layer 113. The grid electrode115 is provided to be in contact with at least part of thesecond-conductivity-type crystalline semiconductor region 111. The gridelectrode 115 is provided in contact with the second-conductivity-typecrystalline semiconductor region 111 with the use of an opening formedin the insulating layer 113, for example.

The grid electrode 115 is a layer formed using a metal element such assilver, copper, aluminum, or palladium by a printing method, a sol-gelmethod, a coating method, an ink-jet method, or the like. By providingthe grid electrode 115 to be in contact with thesecond-conductivity-type crystalline semiconductor region 111,resistance loss can be reduced and electrical characteristics can beimproved, in particular, under high illuminance.

Next, a method for manufacturing the photoelectric conversion deviceillustrated in FIG. 1 will be described with reference to FIGS. 4A to4C.

As in FIG. 4A, the conductive layer 104 is formed over the substrate101. The conductive layer 104 can be formed by a printing method, acoating method, an ink-jet method, a CVD method, a sputtering method, anevaporation method, or the like, as appropriate. Note that in the casewhere the conductive layer 104 has a foil shape, it is not necessary toprovide the substrate 101. Further, roll-to-roll processing can beemployed.

Next, as in FIG. 4B, the first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 are formed by an LPCVD method. The LPCVD method is performed in thefollowing manner at least a deposition gas containing silicon is used asa source gas; the pressure in a reaction chamber of the LPCVD apparatusis set to a pressure lower than or equal to 200 Pa and higher than orequal to the lower limit at which the pressure can be maintained whilethe source gas flows; and heating is performed.

In the LPCVD method, the heating temperature (or a film formationtemperature) is lower than or equal to the temperature which the LPCVDapparatus and the conductive layer 104 can withstand. In the formationof the first-conductivity-type crystalline semiconductor region 107 andthe intrinsic crystalline semiconductor region 109, the temperature ishigher than 550° C. and lower than 650° C., preferably higher than orequal to 580° C. and lower than 650° C. In the formation of thesecond-conductivity-type crystalline semiconductor region 111, thetemperature may be lower than or equal to 550° C. or higher than orequal to 650° C.

By an LPCVD method at a temperature higher than 550° C. and lower than650° C., preferably higher than or equal to 580° C. and lower than 650°C., the first-conductivity-type crystalline semiconductor region 107including whiskers including a crystalline semiconductor or theintrinsic crystalline semiconductor region 109 including whiskersincluding a crystalline semiconductor can be formed. At the same time,the impurity element imparting the first conductivity type is moved fromthe first-conductivity-type crystalline semiconductor region 107 towardthe intrinsic crystalline semiconductor region 109, whereby aconcentration gradient of an impurity element imparting the firstconductivity type is formed from the first-conductivity-type crystallinesemiconductor region 107 toward the intrinsic crystalline semiconductorregion 109.

By an LPCVD method at a temperature lower than or equal to 550° C. orhigher than or equal to 650° C., the second-conductivity-typecrystalline semiconductor region 111 can be formed (i.e. deposited)while whiskers are not grown.

When the second-conductivity-type crystalline semiconductor region 111is formed (i.e. deposited) while whiskers are not grown, an angle ofinclination of the uneven surface including a plurality of whiskers canbe controlled and thus the angle of inclination of the uneven surfacecan be adequately reduced. Thus, the coverage with the insulating layer113 which is formed later can be improved and the reflectance of lighton the surface of the second-conductivity-type crystalline semiconductorregion 111 can be reduced.

When a temperature lower than or equal to 550° C. is employed for theformation of the second-conductivity-type crystalline semiconductorregion 111, it can be formed of an amorphous semiconductor material or asemiconductor material in which an amorphous semiconductor and acrystalline semiconductor are mixed. Alternatively, when a temperaturehigher than or equal to 650° C. is employed, thesecond-conductivity-type crystalline semiconductor region 111 can formedof a crystalline semiconductor.

Examples of the deposition gas containing silicon include siliconhydride, silicon fluoride, and silicon chloride; typically, SiH₄, Si₂H₆,SiF₄, SiCl₄, Si₂Cl₆, and the like are given. Note that hydrogen may beadded to the source gas.

When the first-conductivity-type crystalline semiconductor region 107 isformed by an LPCVD method, the mixed layer 105 may be formed between theconductive layer 104 and the first-conductivity-type crystallinesemiconductor region 107. In a step of forming thefirst-conductivity-type crystalline semiconductor region 107, activespecies of the source gas are constantly supplied to a portion beingdeposited, and silicon diffuses from the first-conductivity-typecrystalline semiconductor region 107 to the conductive layer 104,whereby the mixed layer 105 is formed. In the case where the mixed layer105 is formed, a low-density region (a sparse region) is not easilyformed at an interface between the conductive layer 104 and thefirst-conductivity-type crystalline semiconductor region 107, and thusthe characteristics of the interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 areimproved, whereby series resistance can be reduced.

The first-conductivity-type crystalline semiconductor region 107 isformed by an LPCVD method in which diborane and a deposition gascontaining silicon are introduced into a reaction chamber of an LPCVDapparatus as a source gas and the temperature is higher than 550° C. andlower than 650° C. The thickness of the first-conductivity-typecrystalline semiconductor region 107 is greater than or equal to 5 nmand less than or equal to 500 nm. Here, a crystalline silicon layer towhich boron is added is formed as the first-conductivity-typecrystalline semiconductor region 107.

Then, the introduction of diborane into the reaction chamber of theLPCVD apparatus is stopped. Then, the intrinsic crystallinesemiconductor region 109 is formed by an LPCVD method in which adeposition gas containing silicon is introduced as a source gas into thereaction chamber of the LPCVD apparatus and the temperature is higherthan 550° C. and lower than 650° C. The thickness of the intrinsiccrystalline semiconductor region 109 is greater than or equal to 1 μmand less than or equal to 20 μm. Here, a crystalline silicon layer isformed as the intrinsic crystalline semiconductor region 109. In thisstep, as illustrated in FIG. 6, part of boron (X) included in thefirst-conductivity-type crystalline semiconductor region 107 moves fromthe first-conductivity-type crystalline semiconductor region 107 towardthe intrinsic crystalline semiconductor region 109, whereby aconcentration of boron (X) is increased from the intrinsic crystallinesemiconductor region 109 side toward the first-conductivity-typecrystalline semiconductor region 107 and a concentration gradient isformed.

Then, the second-conductivity-type crystalline semiconductor region 111is formed by an LPCVD method in which phosphine or arsine and adeposition gas containing silicon are introduced as a source gas intothe reaction chamber of the LPCVD apparatus. The thickness of thesecond-conductivity-type crystalline semiconductor region 111 is greaterthan or equal to 5 nm and less than or equal to 500 mn Here, a siliconlayer to which phosphorus or arsenic is added is formed as thesecond-conductivity-type crystalline semiconductor region 111.

Through the above steps, the photoelectric conversion layer 106including the first-conductivity-type crystalline semiconductor region107, the intrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can beformed.

Note that, in the manufacturing process of the photoelectric conversiondevice illustrated in FIG. 1, in the case where the introduction ofdiborane into the reaction chamber of the LPCVD apparatus is stoppedbefore whiskers are formed in the first-conductivity-type crystallinesemiconductor region 107, the interface between thefirst-conductivity-type crystalline semiconductor region 107 and theintrinsic crystalline semiconductor region 109 is flat as in FIG. 1. Onthe other hand, in the case where the introduction of diborane into thereaction chamber of the LPCVD apparatus is stopped after whiskers areformed in the first-conductivity-type crystalline semiconductor region108, the interface between the first-conductivity-type crystallinesemiconductor region 108 and the intrinsic crystalline semiconductorregion 109 is uneven as in FIG. 2. In the step of forming the intrinsiccrystalline semiconductor region 109, as illustrated in FIG. 7, part ofboron (X) included in the crystalline semiconductor region 108 movesfrom the crystalline semiconductor region 108 toward the crystallinesemiconductor region 109, whereby a concentration of boron (X) isincreased from the intrinsic crystalline semiconductor region 109 sidetoward the first-conductivity-type crystalline semiconductor region 108and a concentration gradient is formed.

A surface of the conductive layer 104 may be cleaned with hydrofluoricacid before the formation of the first-conductivity-type crystallinesemiconductor region 107. This step can enhance the adhesiveness betweenthe electrode 103 and the first-conductivity-type crystallinesemiconductor region 107.

Further, nitrogen or a rare gas such as helium, neon, argon, or xenonmay be mixed into the source gas of the first-conductivity-typecrystalline semiconductor region 107, the intrinsic crystallinesemiconductor region 109, and the second-conductivity-type crystallinesemiconductor region 111. In the case where nitrogen or a rare gas isadded to the source gas of the first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111, the density of whiskers can be increased.

After the formation of one or more of the first-conductivity-typecrystalline semiconductor region 107, the intrinsic crystallinesemiconductor region 109, and the second-conductivity-type crystallinesemiconductor region 111, in the case where the introduction of thesource gas into the reaction chamber of the LPCVD apparatus is stoppedand the temperature is maintained in a vacuum state (i.e., vacuumheating is performed), the density of whiskers included in thefirst-conductivity-type crystalline semiconductor region 107 or theintrinsic crystalline semiconductor region 109 can be increased.

Then, as in FIG. 4C, the insulating layer 113 is formed over thesecond-conductivity-type crystalline semiconductor region 111. Theinsulating layer 113 can be formed by a CVD method, a sputtering method,an evaporation method, or the like.

With the above steps, a photoelectric conversion device with a highconversion efficiency can be manufactured even when an electrode havinga texture structure is not formed.

Embodiment 2

In this embodiment, a method for manufacturing a photoelectricconversion layer which has fewer defects than the photoelectricconversion layer in Embodiment 1 is described.

After one or more of the first-conductivity-type crystallinesemiconductor region 107, the first-conductivity-type crystallinesemiconductor region 108, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111, which are described in Embodiment 1, are formed, the temperature ofa reaction chamber in an LPCVD apparatus is set at a temperature higherthan or equal to 400° C. and lower than or equal to 450° C., theintroduction of a source gas into the LPCVD apparatus is stopped, andhydrogen is introduced. Then, in a hydrogen atmosphere, heat treatmentat a temperature higher than or equal to 400° C. and lower than or equalto 450° C. is performed. In this manner, dangling bonds in one or moreof the first-conductivity-type crystalline semiconductor region 107, thefirst-conductivity-type crystalline semiconductor region 108, theintrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can beterminated with hydrogen. The heat treatment is also referred to as ahydrogenation treatment. As a result of the heat treatment, defects inone or more of the first-conductivity-type crystalline semiconductorregion 107, the first-conductivity-type crystalline semiconductor region108, the intrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can bereduced, which leads to less recombination of photoexcited carriers indefects and also leads to an increase in conversion efficiency of thephotoelectric conversion device.

Note that the hydrogenation treatment is preferably performed at leastafter the intrinsic crystalline semiconductor region 109 is formed. Inthat case, the conversion efficiency of the photoelectric conversiondevice can be increased while the throughput is increased.

Embodiment 3

In this embodiment, the structure of a so-called tandem photoelectricconversion device in which a plurality of photoelectric conversionlayers are stacked is described with reference to FIG. 5. Although twophotoelectric conversion layers are stacked in this embodiment, three ormore photoelectric conversion layers may be stacked. In the followingdescription, the photoelectric conversion layer which is closest to thelight incident surface may be referred to as a top cell and thephotoelectric conversion layer which is farthest from the light incidentsurface may be referred to as a bottom cell.

FIG. 5 illustrates a photoelectric conversion device in which thesubstrate 101, the electrode 103, the photoelectric conversion layer 106which is the bottom cell, a photoelectric conversion layer 120 which isthe top cell, and the insulating layer 113 are stacked. Here, thephotoelectric conversion layer 106 includes the first-conductivity-typecrystalline semiconductor region 107, the intrinsic crystallinesemiconductor region 109, and the second-conductivity-type crystallinesemiconductor region 111, which are described in Embodiment 1. Thephotoelectric conversion layer 120 includes a third-conductivity-typesemiconductor region 121, an intrinsic semiconductor region 123, and afourth-conductivity-type semiconductor region 125. The band gap of theintrinsic crystalline semiconductor region 109 in the photoelectricconversion layer 106 is preferably different from that of the intrinsicsemiconductor region 123 in the photoelectric conversion layer 120. Useof semiconductors having different band gaps allows absorption of lightwith a wide range of wavelengths; thus, a photoelectric conversionefficiency can be improved.

For example, a semiconductor with a large band gap can be used for thetop cell while a semiconductor with a small band gap can be used for thebottom cell, and needless to say, vice versa. Here, as an example, astructure where a crystalline semiconductor (typically, crystallinesilicon) is used in the photoelectric conversion layer 106, which is thebottom cell, and an amorphous semiconductor (typically, amorphoussilicon) is used in the photoelectric conversion layer 120, which is thetop cell, is described.

Note that although a structure where light is incident on the insulatinglayer 113 is described in this embodiment, one embodiment of thedisclosed invention is not limited thereto. Light may be incident on therear surface of the substrate 101 (the lower surface in the drawing).

The structures of the substrate 101, the electrode 103, thephotoelectric conversion layer 106, and the insulating layer 113 aresimilar to those in the above embodiments and description thereof isomitted here.

In the photoelectric conversion layer 120, which is the top cell, asemiconductor layer including a semiconductor material to which animpurity element imparting a conductivity type is added is typicallyused as the third-conductivity-type semiconductor region 121 and thefourth-conductivity-type semiconductor region 125. Details of thesemiconductor material and the like are similar to those of thefirst-conductivity-type crystalline semiconductor region 107 inEmbodiment 1. In this embodiment, the case where silicon is used as thesemiconductor material, the third conductivity type is p-type, and thefourth conductivity type is n-type is described. In addition, thecrystallinity of the semiconductor layer is amorphous. It is needless tosay that the third conductivity type may be n-type, the fourthconductivity type may be p-type, and the semiconductor layer is notnecessarily amorphous.

For the intrinsic semiconductor region 123, silicon, silicon carbide,germanium, gallium arsenide, indium phosphide, zinc selenide, galliumnitride, silicon germanium, or the like is used. Alternatively, asemiconductor material including an organic material, a metal oxidesemiconductor material, or the like can be used.

In this embodiment, amorphous silicon is used for the intrinsicsemiconductor region 123. It is needless to say that the intrinsicsemiconductor region 123 may be formed using a semiconductor materialwhich is not silicon and has a band gap different from that of theintrinsic crystalline semiconductor region 109 in the bottom cell. Here,the thickness of the intrinsic semiconductor region 123 is preferablysmaller than that of the intrinsic crystalline semiconductor region 109and is typically greater than or equal to 50 nm and less than or equalto 1000 nm, preferably greater than or equal to 100 nm and less than orequal to 450 nm.

A plasma CVD method, an LPCVD method, or the like may be employed forforming the third-conductivity-type semiconductor region 121, theintrinsic semiconductor region 123, and the fourth-conductivity-typesemiconductor region 125. In the case of a plasma CVD method, theintrinsic semiconductor region 123 can be formed in such a manner thatthe pressure in a reaction chamber of a plasma CVD apparatus istypically greater than or equal to 10 Pa and less than or equal to 1332Pa, hydrogen and a deposition gas containing silicon are introduced as asource gas to the reaction chamber, and high-frequency electric power issupplied to an electrode to cause glow discharge. Thethird-conductivity-type semiconductor region 121 can be formed using theabove source gas to which diborane is added. The third-conductivity-typesemiconductor region 121 is formed with a thickness of greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm. Thefourth-conductivity-type semiconductor region 125 can be formed usingthe above source gas to which phosphine or arsine is added. Thefourth-conductivity-type semiconductor region 125 is formed with athickness of greater than or equal to 1 nm and less than or equal to 100nm, preferably greater than or equal to 5 nm and less than or equal to50 nm.

Alternatively, the third-conductivity-type semiconductor region 121 maybe formed by forming an amorphous silicon layer by a plasma CVD methodor an LPCVD method without adding an impurity element imparting aconductivity type and then adding boron by a method such as ioninjection. The fourth-conductivity-type semiconductor region 125 may beformed by forming an amorphous silicon layer by a plasma CVD method oran LPCVD method without adding an impurity element imparting aconductivity type and then adding phosphorus or arsenic by a method suchas ion injection.

As described above, by using amorphous silicon for the photoelectricconversion layer 120, light having a wavelength of less than 800 nm canbe effectively absorbed and subjected to photoelectric conversion.Further, by using crystalline silicon for the photoelectric conversionlayer 106, light having a longer wavelength (e.g., a wavelength up toapproximately 1200 nm) can be absorbed and subjected to photoelectricconversion. Such a structure (a so-called tandem structure) in whichphotoelectric conversion layers having different band gaps are stackedcan significantly increase a photoelectric conversion efficiency.

Note that although amorphous silicon having a large band gap is used inthe top cell and crystalline silicon having a small band gap is used inthe bottom cell in this embodiment, one embodiment of the disclosedinvention is not limited thereto. The semiconductor materials havingdifferent band gaps can be used in appropriate combination to form thetop cell and the bottom cell. The structure of the top cell and thestructure of the bottom cell can be replaced with each other to form thephotoelectric conversion device. Alternatively, a stacked layerstructure in which three or more photoelectric conversion layers arestacked can be employed.

With the above structure, the conversion efficiency of a photoelectricconversion device can be increased.

Embodiment 4

In this embodiment, an example where a conductive layer is formed by awet process over a second-conductivity-type semiconductor region in aphotoelectric conversion device is described with reference to FIG. 8.

FIG. 8 is a photoelectric conversion device including the substrate 101,the electrode 103, the first-conductivity-type crystalline semiconductorregion 107, the intrinsic crystalline semiconductor region 109, thesecond-conductivity-type crystalline semiconductor region 111, and aconductive layer 213. The second conductivity type is opposite to thefirst conductivity type. The first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 function as the photoelectric conversion layer 106.

The electrode 103 may include the conductive layer 104 and the mixedlayer 105. In addition, an interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 is flat.The intrinsic crystalline semiconductor region 109 includes a pluralityof whiskers (a group of whiskers). Accordingly, the interface betweenthe intrinsic crystalline semiconductor region 109 and thesecond-conductivity-type crystalline semiconductor region 111, and asurface of the second-conductivity-type crystalline semiconductor region111 are uneven.

In this embodiment, the conductive layer 213 is formed by a wet processover part of or the whole of the second-conductivity-type crystallinesemiconductor region 111. Thus, the conductive layer 213 can be formedwith a good coverage over a surface of the second-conductivity-typecrystalline semiconductor region 111 which has an uneven surface owingto the formation of whiskers. By forming the conductive layer 213 by awet process over the surface of the second-conductivity-type crystallinesemiconductor region 111 which has an uneven surface owing to theformation of whiskers, the resistance of the light incident surface canbe reduced. Further, the conductive layer 213 may be used as anelectrode. A material which is used for the conductive layer 213 ispreferably a material which transmits light in a wavelength region whichcan be absorbed by a semiconductor region serving as the photoelectricconversion layer 106.

A wet process can be a coating method such as a dip coating method, aspin coating method, a spray coating method, an ink jet method, or aprinting method. Alternatively, an electrolytic plating method, anelectroless plating method, or the like can be used.

A coating liquid used in a coating method may be a liquid or aliquid-like substance such as a sol or a gel which contains a conductivematerial. The conductive material may be a fine particle of alight-transmitting conductive metal oxide such as indium oxide-tin oxidealloy, zinc oxide, tin oxide, or zinc oxide containing aluminum; a fineparticle of a metal such as gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd), or silver (Ag); or a conductive polymersuch as conductive polyaniline, conductive polypyrrole, conductivepolythiophene, polyethylenedioxythiophene (PEDOT), or polystyrenesulfonate (PSS). In the case where a fine particle is used as theconductive material, the surface of the fine particle may be coated withan organic substance or the like in order to improve dispersibility. Asolvent (or a disperse medium) of the liquid which contains a conductivematerial can be water, alcohols, hydrocarbon-based compounds, ethercompounds, or the like. These solvents (or disperse mediums) may be usedalone or two or more of these solvents may be used in combination.

When a coating method is used as a wet process, a liquid or liquid-likesubstance which contains a conductive material is applied, dried, andbaked, whereby the conductive layer 213 can be formed. When a coatingmethod is used as a wet process, the thickness of the conductive layer213 can be easily increased and thus the resistance of the conductivelayer 213 can be reduced.

When the conductive layer 213 is thick, the surface of the conductivelayer 213 becomes flat. In this case, the surface of the conductivelayer 213 may be processed to be uneven so that reflectance of incidentlight may be reduced and that the characteristics of the photoelectricconversion device may be improved owing to a light-trapping effect.

Before the conductive layer 213 is formed, another conductive layer (notshown) may be formed over part of or the whole of thesecond-conductivity-type crystalline semiconductor region 111. Forexample, before the conductive layer 213 is formed, a conductive layermay be formed of a light-transmitting conductive material such as anindium oxide-tin oxide alloy, zinc oxide, tin oxide, or zinc oxidecontaining aluminum by a dry process such as a CVD method, a sputteringmethod, or an evaporation method. By thus providing such a conductivelayer in advance, the surface of the second-conductivity-typecrystalline semiconductor region 111 can be protected. In addition, bythus providing such a conductive layer in advance, the adhesion betweenthe conductive layer 213 and the second-conductivity-type crystallinesemiconductor region 111 can be improved.

Alternatively, the conductive layer 213 may be a conductive liquid (aliquid containing an electrolyte) which is provided over thesecond-conductivity-type crystalline semiconductor region 111 to fill aspace between whiskers and is used as an electrode. In this case, theconductive liquid can be introduced into a space between the substrate101 and a second substrate which faces the substrate 101, and thensealed with a sealant to form the conductive layer 213. In any case, byproviding an electrode to fill a space between whiskers, the resistanceof the light incident surface can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate.

EXAMPLE

In this example, difference in regular reflectance between a sample thatis a titanium foil and a sample including a titanium foil and a group ofwhiskers formed of polysilicon on the titanium foil is described.

First, a method for forming the samples is described. <Sample 1

As the sample 1, a titanium foil with a thickness of 0.1 mm which wascut in a circle with a diameter Φ of 12 mm was used.

Sample 2

A polysilicon layer including a group of whiskers was formed by an LPCVDmethod on a titanium foil with a shape similar to that of the sample 1,i.e. a thickness of 0.1 mm and a diameter Φ of 12 mm. The polysiliconlayer was formed by introducing silane for deposition for 2 hours and 15minutes at a flow rate of 300 sccm into a process chamber in which thepressure was set to 13 Pa and the substrate temperature was set to 600°C.

FIG. 9 illustrates the measurement results of the regular reflectance ofthe samples 1 and 2 with a spectrophotometer (U-4100 Spectrophotometer,manufactured by Hitachi High-Technologies Corporation). Here, thesamples 1 and 2 were irradiated with light having wavelengths from 200nm to 1200 nm with a sampling interval of 2 nm. An incident angle of thelight incident on the samples was 5° and the reflectance of the light(i.e., 5-degree regular reflectance) was measured. The reflectance ofthe sample 1 is shown by a broken line 501 and the reflectance of thesample 2 is shown by a solid line 502. The horizontal axis representsthe wavelength of the irradiation light and the vertical axis representsthe reflectance of the irradiation light.

According to FIG. 9, the light reflectance of the sample 2 in which thepolysilicon layer including the group of whiskers is formed on thesurface of the titanium foil is extremely low, 0.14% at most, whichmeans that there is almost no reflection of light. Note that since thesignal-to-noise ratio (SN ratio) is small in a wavelength range of 850nm to 894 nm, the reflectance is negative. In contrast, the sample 1that is the titanium foil has a regular reflectance of 2% to 15%. Theabove results show that the reflectance can be reduced by forming thepolysilicon layer including the group of whiskers on the surface of thetitanium foil.

This application is based on Japanese Patent Application serial no.2010-140005 filed with Japan Patent Office on Jun. 18, 2010, the entirecontents of which are hereby incorporated by reference.

1. A method for manufacturing a photoelectric conversion device,comprising the steps of: forming a first-conductivity-type crystallinesemiconductor region by a low pressure chemical vapor deposition methodusing a deposition gas containing silicon and a gas imparting the firstconductivity type as a source gas at a temperature higher than 550° C.and lower than 650° C. over a conductive layer; forming an intrinsiccrystalline semiconductor region that includes a crystallinesemiconductor region and a plurality of whiskers including a crystallinesemiconductor by a low pressure chemical vapor deposition method using adeposition gas containing silicon as a source gas at a temperaturehigher than 550° C. and lower than 650° C. over thefirst-conductivity-type crystalline semiconductor region, and alsomoving an impurity element imparting the first conductivity type fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region; and forming asecond-conductivity-type crystalline semiconductor region by a lowpressure chemical vapor deposition method using a deposition gascontaining silicon and a gas imparting the second conductivity type as asource gas at a temperature lower than or equal to 550° C. or higherthan or equal to 650° C. over the intrinsic crystalline semiconductorregion.
 2. The method for manufacturing a photoelectric conversiondevice, according to claim 1, further comprising the step of: forming amixed layer between the conductive layer and the first-conductivity-typecrystalline semiconductor region, wherein the mixed layer is formedusing silicon and a metal element included in the conductive layer. 3.The method for manufacturing a photoelectric conversion device,according to claim 1, wherein silicon hydride, silicon fluoride, orsilicon chloride is used for the deposition gas containing silicon. 4.The method for manufacturing a photoelectric conversion device,according to claim 1, wherein the first-conductivity-type crystallinesemiconductor region is one of an n-type semiconductor region and ap-type semiconductor region, and wherein the second-conductivity-typecrystalline semiconductor region is the other of the n-typesemiconductor region and the p-type semiconductor region.
 5. The methodfor manufacturing a photoelectric conversion device, according to claim1, wherein the gas imparting the first conductivity type is one ofdiborane and phosphine, and wherein the gas imparting the secondconductivity type is the other of the diborane and the phosphine.
 6. Amethod for manufacturing a photoelectric conversion device, comprisingthe steps of: forming a first-conductivity-type crystallinesemiconductor region that includes a crystalline semiconductor regionand a plurality of whiskers including a crystalline semiconductor by alow pressure chemical vapor deposition method using a deposition gascontaining silicon and a gas imparting the first conductivity type as asource gas at a temperature higher than 550° C. and lower than 650° C.over a conductive layer; forming an intrinsic crystalline semiconductorregion by a low pressure chemical vapor deposition method using adeposition gas containing silicon as a source gas at a temperaturehigher than 550° C. and lower than 650° C. over thefirst-conductivity-type crystalline semiconductor region, and alsomoving an impurity element imparting the first conductivity type fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region; and forming asecond-conductivity-type crystalline semiconductor region by a lowpressure chemical vapor deposition method using a deposition gascontaining silicon and a gas imparting the second conductivity type as asource gas at a temperature lower than or equal to 550° C. or higherthan or equal to 650° C. over the intrinsic crystalline semiconductorregion.
 7. The method for manufacturing a photoelectric conversiondevice, according to claim 6, further comprising the step of: forming amixed layer between the conductive layer and the first-conductivity-typecrystalline semiconductor region, wherein the mixed layer is formedusing silicon and a metal element included in the conductive layer. 8.The method for manufacturing a photoelectric conversion device,according to claim 6, wherein silicon hydride, silicon fluoride, orsilicon chloride is used for the deposition gas containing silicon. 9.The method for manufacturing a photoelectric conversion device,according to claim 6, wherein the first-conductivity-type crystallinesemiconductor region is one of an n-type semiconductor region and ap-type semiconductor region, and wherein the second-conductivity-typecrystalline semiconductor region is the other of the n-typesemiconductor region and the p-type semiconductor region.
 10. The methodfor manufacturing a photoelectric conversion device, according to claim6, wherein the gas imparting the first conductivity type is one ofdiborane and phosphine, and wherein the gas imparting the secondconductivity type is the other of the diborane and the phosphine.
 11. Amethod for manufacturing a photoelectric conversion device, comprisingthe steps of: forming a first-conductivity-type crystallinesemiconductor region over a conductive layer; forming an intrinsiccrystalline semiconductor region including a crystalline semiconductorregion and a plurality of whiskers including a crystalline semiconductorover the first-conductivity-type crystalline semiconductor region, andalso moving an impurity element imparting the first conductivity typefrom the first-conductivity-type crystalline semiconductor region towardthe intrinsic crystalline semiconductor region; and forming asecond-conductivity-type crystalline semiconductor region over theintrinsic crystalline semiconductor region.
 12. The method formanufacturing a photoelectric conversion device, according to claim 11,wherein the second-conductivity-type crystalline semiconductor regiondoes not include whisker.
 13. The method for manufacturing aphotoelectric conversion device, according to claim 11, furthercomprising the step of: forming a mixed layer between the conductivelayer and the first-conductivity-type crystalline semiconductor region,wherein the mixed layer is formed using silicon and a metal elementincluded in the conductive layer.
 14. The method for manufacturing aphotoelectric conversion device, according to claim 11, wherein siliconhydride, silicon fluoride, or silicon chloride is used for thedeposition gas containing silicon.
 15. The method for manufacturing aphotoelectric conversion device, according to claim 11, wherein thefirst-conductivity-type crystalline semiconductor region is one of ann-type semiconductor region and a p-type semiconductor region, andwherein the second-conductivity-type crystalline semiconductor region isthe other of the n-type semiconductor region and the p-typesemiconductor region.
 16. The method for manufacturing a photoelectricconversion device, according to claim 11, wherein the gas imparting thefirst conductivity type is one of diborane and phosphine, and whereinthe gas imparting the second conductivity type is the other of thediborane and the phosphine.
 17. A method for manufacturing aphotoelectric conversion device, comprising the steps of: forming afirst-conductivity-type crystalline semiconductor region that includes acrystalline semiconductor region and a plurality of whiskers including acrystalline semiconductor over a conductive layer; forming an intrinsiccrystalline semiconductor region over the first-conductivity-typecrystalline semiconductor region, and also moving an impurity elementimparting the first conductivity type from the first-conductivity-typecrystalline semiconductor region toward the intrinsic crystallinesemiconductor region; and forming a second-conductivity-type crystallinesemiconductor region over the intrinsic crystalline semiconductorregion.
 18. The method for manufacturing a photoelectric conversiondevice, according to claim 17, wherein the second-conductivity-typecrystalline semiconductor region does not include whisker.
 19. Themethod for manufacturing a photoelectric conversion device, according toclaim 17, further comprising the step of: forming a mixed layer betweenthe conductive layer and the first-conductivity-type crystallinesemiconductor region, wherein the mixed layer is formed using siliconand a metal element included in the conductive layer.
 20. The method formanufacturing a photoelectric conversion device, according to claim 17,wherein silicon hydride, silicon fluoride, or silicon chloride is usedfor the deposition gas containing silicon.
 21. The method formanufacturing a photoelectric conversion device, according to claim 17,wherein the first-conductivity-type crystalline semiconductor region isone of an n-type semiconductor region and a p-type semiconductor region,and wherein the second-conductivity-type crystalline semiconductorregion is the other of the n-type semiconductor region and the p-typesemiconductor region.
 22. The method for manufacturing a photoelectricconversion device, according to claim 17, wherein the gas imparting thefirst conductivity type is one of diborane and phosphine, and whereinthe gas imparting the second conductivity type is the other of thediborane and the phosphine.